One treatment approach in skeletal malocclusions is the use of external mechanical forces, but their effectiveness in influencing midfacial growth is much debated. This is not due to a conceptual fallacy, since the ability of mechanical stimuli to affect bone is well documented. Instead, a main reason is that tooth movement rather than skeletal change is a primary consequence of conventional force systems which use the teeth as point of force application. Another significant factor is the lack of precise data on how skeletal tissue response alters with variation in mechanical stress, quantitatively defined at sutural target areas. These problems are focuses of the present proposal which has two main objectives: (a) Firstly, to evaluate the efficacy of delivering extraoral forces directly to the midfacial skeleton via intrabony pin-retained appliances, thus eliminating the dentition as mediator in the force system. Using the growing squirrel monkey as experimental model, protractive and retractive forces will be transmitted to the maxilla through an intraoral appliance retained by means of intraosseous pins. Special emphasis will be centered on (1) degree of skeletal change relative to dentoalveolar and occlusal alteration, (2) adaptation of bone-muscle interface, and (3) degree of migration of intraosseous pins during force application. Methodology of analysis will involve roentgenographic cephalometry using metallic bone markers, in vivo bone labeling for examination of bone remodeling patterns, wet specimens for evaluation of muscle fiber direction, and dry skull specimens for visual interpretation of gross morphological change. (b) The second broad objective is to relate physical characteristics of mechanical forces, acting directly on an experimental site, with (1) differentiation of progenitor cells into particular pathways. (2) cellular transformations, and (3) timing and degree of ossification within the site. Controlled, and locally defined, pressure and tension forces of varying mode, magnitude, and duration will be delivered to cranial sutures in young growing rabbits. The experimental methodology includes partial osteotomies in the sutural areas, elastic modules to exert either compression or tension, and micro-transducers to monitor magnitude of force. The tissue response to each set of mechanical parameters will be analyzed with histological and histochemical methods.